This time, Keys will play No. 20 Naomi Osaka on Thursday night for a chance to reach the final again.

The other men's quarterfinal features No. 6 seed Novak Djokovic against John Millman. "Juan Martin is a great player everywhere", said Nadal who will be playing in a seventh semi-final in NY.

The 20-year-old Osaka, who has looked comfortable on one of the sport's biggest stages, was tearful after her fourth-round clash but much less emotional after her latest win.

The defending champion and No. 1 seed at Flushing Meadows recovered from his disastrous start and other stumbles along the way to beat No. 9 Dominic Thiem 0-6, 6-4, 7-5, 6-7 (4), 7-6 (5) for a semifinal berth at a third consecutive Grand Slam tournament, winning a physical, back-and-forth tussle that concluded after 2 a.m. on Wednesday.

"I've gotten a lot better managing my emotions once it gets to this part and knowing that everything is going to be probably more amped up", Keys said.

"I wish I don't go to five sets every time, " Nishikori said.

The Swiss superstar, owner of 20 Grand Slam titles, said the suffocating humidity on Arthur Ashe Stadium played a role in the defeat.

The momentum swung throughout the match with 2014 champion Cilic looking in control when leading 4-2 in the second set before losing six games in a row. The bottom half of the draw also had former champion Marin Cilic and, therefore, Nishikori was an unforeseen player in the semi-finals, unlike the top half of the draw, where Rafael Nadal will be playing the World no. 3 Del Potro, according to predictions.

The Aussie saved another four in the opening game of the second set and as the duel continued both players were clearly feeling the effects of their long, entertaining rallies.

Should Williams beat Osaka - the first Japanese woman to advance to a Grand Slam final - she will equal Margaret Court's record of 24 Grand Slam singles titles.

Having recovered from 3-1 down to lead 4-3 in the breaker, Cilic double-faulted twice in succession and another missed first serve proved costly at set point down as Nishikori pounced on the second delivery with a backhand return victor. Now, she needs to do it one more time to advance to her first career final.

Earlier, Nishikori defeated the man he lost to in the final four years ago, Marin Cilic 2-6 6-4 7-6 (5) 4-6 6-4.

'We had some matches where I won in straight sets, but we had a lot of matches where it went the distance where a couple of points really decided the victor.

Djokovic, who was a little tired at the time, was more than happy to take a break which turned out to be about six minutes.

"I was struggling, he was struggling", Djokovic added. "I am really happy to win and to go through matches like this".

But Djokovic broke back in the match's next-to-last game, then served out the victory at love.

There were other issues for him, too, including in the third set when, ahead by a break, he was called by the chair umpire for allowing the 25-second serve clock, making its Grand Slam debut at this tournament, to run out on back-to-back points.

"Especially after coming from injury", he said, "I think I'm enjoying this challenge".